1. Field of the Invention
The present invention relates to an apparatus for shifting the phase between shafts (hereinafter referred to as intershaft phase shifting apparatus). More specifically, this invention pertains to an intershaft phase shifting apparatus for a valve timing control device or the like which shifts the rotational phase of a cam shaft to a crank shaft and varying the amount of the phase shift in order to control the opening/closing timing of intake and exhaust valves in an internal combustion engine in accordance with the running state of the engine.
2. Description of the Related Art
As a phase shifting apparatus of this type, a phase shifting apparatus (cam shaft driving apparatus) using a planetary gear mechanism is disclosed in, for example, Japanese Unexamined Utility Model Publication No. 59-156102. According to this technique, a cam shaft 61 is divided into an input-side shaft 61a for driving a crank shaft 62 and an output-side shaft 61b having a cam 63 as shown in FIG. 10. A planetary gear mechanism 64 intervenes between these shafts 61a and 61b. In the planetary gear mechanism 64, a carrier 65 is attached to the end of the input-side shaft 61a. On the carrier 65 are rotatably supported a plurality of planetary gears 66 whose outer peripheral portions are engaged with a ring gear 67. A sun gear 68 is attached to the end of the output-side shaft 61b, and it is engaged with the inner peripheral portions of the planetary gears 66. The ring gear 67 is rotated by a piston 70 which is thrust forward and backward from a cylinder 69.
According to this apparatus, as the ring gear 67 rotates due to the forward/backward movement of the piston 70, the carrier 65 and sun gear 68 rotate by different angles. This causes the rotational phase of the crank shaft 62 to deviate from that of the output-side shaft 61b, thus changing the opening/closing timing of the intake and exhaust valves during engine running. The amount of a phase shift is varied by an amount corresponding to the ratio of the diameter of the sun gear 68 to that of the planetary gears 66.
According to the conventional phase shifting apparatus, however, when no phase shift is performed, the piston 70 will not be activated and the ring gear 67 stays unmoved. It is therefore necessary to always rotate the planetary gears 66 and the sun gear 68. The backlash between those gears 66, 67 and 68 may cause gearing noise or wear out the gears.